The invention relates to the preparation of poly(diorganosiloxanes) with diorganooxy-organosilyl or triorganooxysilyl end groups by reaction of .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes in the presence of catalytically active ammonium carbamates.
Several processes are already known for the preparation of the polymers according to the invention or of formulations containing such polymers. The products are used for the preparation of one-component silicone pastes which harden into elastomers in the presence of moisture, hereinafter termed "1C RTV pastes".
In U.S. Pat. No. 3,161,614 the reaction of .alpha.,.omega.-dihydroxy(polydiorgano)siloxanes with multifunctional halosilanes, e.g., SiCl.sub.4 or CH.sub.3 SiCl.sub.3 is described. The halogen-containing polysiloxanes resulting therefrom are then converted into di- or triorganooxy silyl-terminated polysiloxanes by alcohols in the presence of acid scavengers.
In the same patent the reaction of .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes in the presence of suitable catalysts such as amines and metal carboxylates is also mentioned.
In European Patent Specification No. 21859 and in European Patent Specification No. 69256 the preparation of 1C RTV pastes is described. According to these publications .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) can be reacted with alkoxysilanes in the presence of amines to form the polymers according to the invention.
In European Patent Specification No. 70786 the use of hydroxylamine derivatives instead of amines as catalysts is described.
Furthermore, mixed-functional silanes have become known which, in contrast to pure organyloxy or organyloxyorganosilanes, can be reacted with .alpha.,.omega.-dihydroxylpoly(diorganosiloxanes) even without using catalysts to form the polysiloxanes prepared according to the invention. These include alkoxyamidosilanes (German Patent Specification No. 1 247 646), alkoxyoximinosilanes (European Patent Specification No. 98369 and alkoxyacetoxysilanes (U.S. Pat. No. 3,296,195).
All the processes described hitherto for the preparation of the polysiloxanes mentioned have disadvantages. The process of reacting .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) with halosilanes with subsequent alcoholysis (U.S. Pat. No. 3,161,614) results in polymers which contain ammonia salts with a corrosive action and is long-winded. The success of the process mentioned in the same document of reacting .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes in the presence of amine catalysts depends essentially on the base strength of the catalyst used. The catalytic effectiveness of the amines increases with the base strength, but so also does the tendency for undesired polymer rearrangements. Thus, for example, it is known for strong bases such as KOH or potassium siloxanolates (U.S. Pat. No. 2,909,549) that with cyclotetra(dimethylsiloxane) they result in monoorganyloxy-terminated poly(diorganosiloxanes). In a similar manner, from .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) and alkoxysilanes in the presence of KOH branched polysiloxanes are rapidly produced which have monoorganyloxydiorganosilyl end groups. No serviceable 1C RTV pastes can be prepared from such polysiloxanes. In the case of amines there is apparently a restriction to mono- and dialkylamines which represent a compromise between the activity of the catalyst in relation to the desired termination process and the undesired rearrangement. However, in the presence of dialkylamines, the reaction of the OH end groups only proceeds sufficiently rapidly if particularly reactive silanes are employed. Otherwise the formulations must be kept for a rather long time at elevated temperature, which represents a disadvantage in the practical application of the preparation of 1C RTV pastes.
The catalysis mentioned in U.S. Pat. No. 2,909,549 of the reaction of .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes by metal carboxylates such as diorganotin dicarboxylates, has proved quite unsuitable for the practical application of preparing 1C RTV pastes since pastes prepared in this manner exhibit crosslinking phenomena during the preparation or storage of the products.
Although the hydroxylamine derivatives described as catalysts in European Patent Specification 70786 have an improved activity compared with mono- and dialkylamines, in many cases, however, they result, after formulation to form 1C RTV systems, in coloration phenomena and have to be removed from the system.
Mixed-functional alkoxysilanes which carry an amido, amino, oximino or carboxylate radical in addition to the alkoxy groups yield, with .alpha.,.omega.-dihydroxypoly(diorganosiloxanes) precisely the desired poly(diorganosiloxanes) with triorganooxysilyl or diorganooxyorganosilyl end groups. The preparation of the silanes is, however, usually expensive and the removal of the fission products makes special process steps necessary in the preparation of the formulations or cannot be carried out in practice. The removal of the fission products resulting from the silanes mentioned would, however, be desirable, for example for the formulation of chemically neutral, transparent systems.